cmake: Improve QtGStreamerConfig.cmake
cmake: Improve QtGStreamerConfig.cmake

* Honor QUIET
* Fail when dependencies are not found, without using the nasty REQUIRED keyword
* Unset useless variables when done
* Do not require Qt5Widgets as a dependency

freedesktop-unofficial-mirror/gstreamer__qt-gstreamer,tswindell/qt-gstreamer,nasserGanjali/QtGStreamer,dejunk/gstreamer,detrout/qt-gstreamer,tswindell/qt-gstreamer,iperry/qt-gstreamer,nasserGanjali/QtGStreamer,GStreamer/qt-gstreamer,dejunk/gstreamer,GStreamer/qt-gstreamer,detrout/qt-gstreamer,knuesel/qt-gstreamer,freedesktop-unofficial-mirror/gstreamer__qt-gstreamer,iperry/qt-gstreamer,dejunk/gstreamer,knuesel/qt-gstreamer,GStreamer/qt-gstreamer,detrout/qt-gstreamer,tswindell/qt-gstreamer,knuesel/qt-gstreamer,freedesktop-unofficial-mirror/gstreamer__qt-gstreamer,nasserGanjali/QtGStreamer,iperry/qt-gstreamer